Flat panel display with thin film transistor

ABSTRACT

A flat panel display capable of lowering an on-current of a driving thin film transistor (TFT), maintaining high switching properties of a switching TFT, maintaining uniform brightness using the driving TFT, and maintaining a life span of a light emitting device while the same voltages are applied to the switching TFT and the driving TFT without changing a size of an active layer. The flat panel display includes a light emitting device, a switching thin film transistor including a semiconductor active layer having a channel area for transferring a data signal to the light emitting device, and a driving thin film transistor including a semiconductor active layer having a channel area for driving the light emitting device. A predetermined amount of current flows through the light emitting device according to the data signal. The channel area of the switching thin film transistor has crystal grains with at least one of different sized or different shaped crystal grains than the crystal grains in the channel area of the driving thin film transistor.

BACKGROUND OF THE INVENTION

[0001] This application claims the priority of Korean Patent ApplicationNo. 2003-20738, filed on Apr. 2, 2003, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

[0002] 1. Field of the Invention

[0003] The invention relates to an active matrix type flat panel displayincluding a thin film transistor (TFT), and more particularly, to a flatpanel display including a TFT having a polycrystalline silicon as anactive layer, and different crystallization structures for the channelareas of the active layers of a switching TFT and a driving TFT.

[0004] 2. Description of the Related Art

[0005] A thin film transistor (TFT) in a flat display device such as aliquid display device, an organic electroluminescence display device, oran inorganic electroluminescence display device is used as a switchingdevice for controlling operations of pixels and as a driving device fordriving the pixels.

[0006] The TFT includes a semiconductor active layer having a drain areaand a source area which are doped with a high concentration ofimpurities and a channel area formed between the drain area and thesource area, a gate insulating layer formed on the semiconductor activelayer, and a gate electrode formed on the gate insulating layer which islocated on an upper part of the channel area of the active layer. Thesemiconductor active layer can be classified as an amorphous silicon ora polycrystalline silicon according to the crystallized status of thesilicon.

[0007] A TFT using amorphous silicon is advantageous in that adeposition can be performed at a low temperature, however, it isdisadvantageous in that an electrical property and a reliability of theTFT are degraded. Also, it is difficult to make larger display devices.Thus, recently, polycrystalline silicon is being used. Polycrystallinesilicon has a higher mobility of about tens to hundreds of cm²/V.s, andlow high frequency operation property and leakage current value. Thus,polycrystalline silicon is suitable for use in large-sized flat paneldisplays of high resolution.

[0008] A TFT is used as the switching device or the driving device ofthe pixel in the flat panel display, as described above. An organicelectroluminescence display device of an active matrix type with anactive driving method includes at least two TFTs per sub-pixel.

[0009] The organic electroluminescence device has an emission layer madeof an organic material between an anode electrode and a cathodeelectrode. In the organic electroluminescence device, when a positivevoltage and a negative voltage are respectively applied to theelectrodes, holes injected from the anode electrode are moved to theemission layer through a hole transport layer, and electrons areinjected into the emission layer through an electron transport layerfrom the cathode electrode. The holes and electrons are recombined onthe emission layer to produce exitons. The exitons are changed from anexcited status to a ground status, and accordingly, phosphor moleculesin the emission layer are radiated to form an image. In case of afull-color electroluminescence display, pixels radiating red (R), green(G), and blue (B) colors are disposed as electroluminescence devices torealize the full colors.

[0010] In the active matrix type organic electroluminescence displaydevice, a panel with high resolution is required, however, the abovedescribed TFT formed using the polycrystalline silicon of high functioncauses some problems in this case.

[0011] That is, in the active matrix type flat panel display device suchas the active matrix type organic electroluminescence display device,the switching TFT and the driving TFT are made of the polycrystallinesilicon. Thus, the switching TFT and the driving TFT have the samecurrent mobility. Therefore, switching properties of the switching TFTand low current driving properties of the driving TFT cannot besatisfied simultaneously. That is, when the driving TFT and theswitching TFT of a high resolution display device are fabricated usingthe polycrystalline silicon, which has a having larger current mobility,the high switching property of the switching TFT can be obtained,however, the brightness becomes too bright because an amount of currentflowing toward an electroluminescence (EL) device through the drivingTFT increases. Thus a current density per unit area of the device isincreased while a life time of the EL device is decreased.

[0012] On the other hand, when the switching TFT and the driving TFT ofthe display device are fabricated using the amorphous silicon, which hasa low current mobility, the TFTs should be fabricated in such way thatthe driving TFT uses a small current and the switching TFT uses a largecurrent.

[0013] To solve the above problems, methods for restricting currentflowing through the driving TFT are provided, such as, a method forincreasing resistance of a channel area by reducing a ratio of a lengthto a width of the driving TFT (W/L) and a method for increasingresistance by forming a low doped area on the source/drain areas of thedriving TFT.

[0014] However, in the method decreasing the W/L by increasing thelength, a length of the channel area increases, thus forming stripes onthe channel area and reducing an aperture area in a crystallizationprocess in an excimer laser annealing (ELA) method. The methoddecreasing W/L by reducing the width is limited by a design rule of aphotolithography process, and it is difficult to ensure a reliability ofthe TFT.

[0015] Also, the method for increasing the resistance by forming the lowdoped area requires an additional doping process.

[0016] A method for increasing TFT properties by reducing a thickness ofthe channel area is disclosed in U.S. Pat. No. 6,337,232.

[0017] The method for reducing a ratio of a length for a width of thedriving TFT is disclosed in Japanese Patent Publication No. 2001-109399.

SUMMARY OF THE INVENTION

[0018] The invention provides a flat panel display in which anon-current of a driving thin film transistor (TFT) is lowered whilekeeping constant a driving voltage applied thereto, without changing asize of an active layer of the TFT.

[0019] The invention separately provides a flat panel display capable ofmaintaining high switching properties of a switching TFT, satisfyinguniform brightness by a driving TFT, and maintaining a life span of alight emitting device.

[0020] According to an aspect of the invention, there is provided a flatpanel display device comprising a light emitting device, a switchingthin film transistor including a semiconductor active layer having achannel area for transferring a data signal to the light emittingdevice, and a driving thin film transistor including a semiconductoractive layer having at least a channel area for driving the lightemitting device so that a predetermined amount of current flows throughthe light emitting device according to the data signal, the channelareas of the switching thin film transistor having crystal grains withat least one of a different size and a different shape than the crystalgrains in the channel area of the driving thin film transistor.

[0021] In various embodiments of the invention, the current mobilitiesin the channel areas of the switching TFT and the driving TFT aredifferent from each other due to the shapes of crystal grain shapesassociated with each.

[0022] In various embodiments of the invention, the current mobility inthe channel area of the switching TFT may be larger than that in thechannel area of the driving TFT due to the crystal grain shapes on thechannel areas.

[0023] In various embodiments of the invention, the channel area of theswitching TFT have crystal grains with a size different than a size ofthe crystal grains in the channel area of the driving TFT.

[0024] In various embodiments of the invention, the current mobility inthe channel area of the switching TFT may be larger than that in thecurrent mobility in channel area of the driving TFT due to the sizes ofcrystal grains associated with each.

[0025] In various embodiments of the invention, the size of crystalgrains in the channel area of TFT requiring larger current mobilitybetween the switching TFT and the driving TFT, may be larger than thesize of the crystal grains in the channel area of the other TFT.

[0026] In various embodiments of the invention, the size of crystalgrain on the channel area of the switching TFT may be larger than thesize of the crystal grains in the channel area of the driving TFT.

[0027] In various embodiments of the invention, The channel areas of theswitching TFT and the driving TFT may have differently shaped crystalgrains.

[0028] Between the switching TFT and the driving TFT, the channel areaof TFT requiring lower current mobility may have grain boundaries of anamorphous shape.

[0029] In various embodiments of the invention, the crystal grains inthe channel area of TFT requiring a larger current mobility than that ofTFT having the amorphous grain boundary may include substantiallyparallel primary grain boundaries, and secondary grain boundariesextending substantially perpendicularly from the primary grainboundaries between the primary grain boundaries, and the primary grainboundaries may be formed as stripes or squares.

[0030] In various embodiments of the invention, the crystal grains inthe channel area of TFT requiring higher current mobility between theswitching TFT and the driving TFT may include substantially parallelprimary grain boundaries, and secondary grain boundaries which extendsubstantially perpendicularly from between the primary grain boundariesand are arranged and an average interval between them is shorter than anaverage interval between primary grain boundaries, the primary grainboundaries may be formed to have stripe shapes, and the channel areasmay be arranged so that a direction of current flow is substantiallyperpendicular to the primary grain boundaries.

[0031] In various embodiments of the invention, The channel area of TFTrequiring a lower current mobility than that of TFT having the primarygrain boundaries of stripe shapes may have grain boundaries of amorphousshapes and/or grain boundaries having primary grain boundaries ofsubstantially square shapes.

[0032] In various embodiments of the invention, between the switchingTFT and the driving TFT, the crystal grains in the channel area of TFTrequiring higher current mobility may include substantially parallelprimary grain boundaries, and secondary grain boundaries extendingsubstantially perpendicularly between the primary grain boundaries, andthe primary grain boundaries may be formed to be substantially squareshapes.

[0033] In various embodiments of the invention, the crystal grains inthe channel area of the driving TFT may have grain boundaries of anamorphous shape.

[0034] In various embodiments of the invention, the crystal grains inthe channel area of the switching TFT may have substantially parallelprimary grain boundaries and secondary grain boundaries extendingsubstantially perpendicularly from the primary grain boundaries betweenthe primary grain boundaries, and the primary grain boundaries may beformed as stripes or squares.

[0035] In various embodiments of the invention, the crystal grains onthe channel area of the switching TFT may have substantially parallelprimary grain boundaries and secondary grain boundaries extendingsubstantially perpendicularly from the primary grain boundaries betweenthe primary grain boundaries, and the primary grain boundaries may beformed substantially as striped shapes.

[0036] In various embodiments of the invention, the crystal grains inthe channel area of the driving thin film transistor may have grainboundaries of an amorphous shape and/or having primary grain boundariesof substantially square shapes.

[0037] In various embodiments of the invention, the crystal grains onthe channel area of the switching thin film transistor may havesubstantially parallel primary grain boundaries and secondary grainboundaries extending substantially perpendicularly from the primarygrain boundaries between the primary grain boundaries, and the primarygrain boundaries may be formed as substantially square shapes.

[0038] In various embodiments of the invention, the channel area of theactive layer may be formed using a polycrystalline silicon, and thepolycrystalline silicon may be formed using a crystallization methodusing a laser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The above and other features of the invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the attached drawings.

[0040]FIG. 1 is a plane view of an active layer structure of a thin filmtransistor (TFT) in an active matrix type organic electroluminescencedisplay according to an exemplary embodiment of the invention.

[0041]FIG. 2 is a plane view of crystalline structures having differentshapes from each other in a polycrystalline silicon thin film formingthe active layer of the TFT.

[0042]FIG. 3 is a graph showing a relation between an angle of primarygrain boundaries for a length of a channel area and a current mobilityon the channel area.

[0043]FIG. 4 is a graph of a ratio between current mobilities ofrespective active layers in a case where the TFT is formed on thedifferent crystallization structures of FIG. 2.

[0044]FIG. 5 is a plane view of a state where a first active layer isformed on a first crystallization structure and a second active layerformed on a second crystallization layer.

[0045]FIG. 6 is a plane view showing that the first active layer isformed on the first crystallization structure and the second activelayer is formed on a third crystallization structure.

[0046]FIG. 7 is a plane view of a status that the first active layer isformed on the second crystallization structure and the second activelayer is formed on the third crystallization structure.

[0047]FIG. 8 is a plane view of a status that the first active layer andthe second active layer are formed on a polycrystalline silicon thinfilm having crystallization structures of different sizes.

[0048]FIG. 9 is a graph of a relation between an energy density and asize of crystal grain in an excimer laser annealing (ELA)crystallization method.

[0049]FIG. 10 is a graph of a relation between a size of crystal grainand a current mobility.

[0050]FIG. 11 is a partially enlarged plane view of a sub-pixel of apixel shown in of FIG. 1.

[0051]FIG. 12 is an equivalent circuit diagram of a unit pixel shown inFIG. 11.

[0052]FIG. 13 is a cross-sectional view of line IV-IV direction in FIG.11.

[0053]FIG. 14 is a cross-sectional view of line V-V direction in FIG.11.

DETAILED DESCRIPTION OF THE INVENTION

[0054]FIG. 1 is a plane view of an active layer structure of a thin filmtransistor (TFT) in an active matrix type organic electroluminescencedisplay according to an exemplary embodiment of the invention. As shownin FIG. 1, red (R), green (G), and blue (B) sub-pixels are repeatedlyarranged in a longitudinal direction (up-and-down direction) in thepixels of the organic electroluminescence display. However, thearrangement of the pixels is not limited to the above structure, and thesub-pixels of respective colors can be arranged in various patterns,such as, a mosaic pattern, or a grid type pattern to construct thepixel. Also, a mono-color flat panel display can be used instead of thefull-color flat panel display shown in FIG. 1.

[0055] In the organic electroluminescence display, a plurality of gatelines 51 are arranged in a transverse direction (left-and-rightdirection), and a plurality of data lines 52 are arranged in alongitudinal direction. Also, driving lines 53 for supplying drivingvoltages (Vdd) are arranged in the longitudinal direction. The gate line51, the data line 52, and the driving line 53 are disposed to surroundone sub-pixel.

[0056] In above construction, each sub-pixel of the R, G, and B pixelsincludes at least two TFTs such as a switching TFT and a driving TFT.The switching TFT transfers a data signal to a light emitting deviceaccording to a signal of the gate line 51 to control operations of thelight emitting device, and the driving TFT drives the light emittingdevice so that a predetermined current flows on the light emittingdevice based on the data signal. The number of TFTs and the arrangementof TFTs, such as, the arrangement of the switching TFT and the drivingTFT can be varied based on the properties of the display device and adriving method of the display device, and TFTs can be arranged invarious ways.

[0057] The switching TFT 10 and the driving TFT 20 include a firstactive layer 11 and a second active layer 21 respectively. Semiconductoractive layers, and the active layers 11 and 21 include channel areas(not shown) which will be described later. The channel areas are theareas located on center portions of the first active layer 11 and thesecond active layer 21 in a current flowing direction.

[0058] As shown in FIG. 1, in sub-pixels forming the R, G, and B pixels,the first active layer 11 included in the switching TFT 10 and thesecond active layer 21 included in the driving TFT 20 can be formed suchthat the first active layer 11 and the second active layer 21 havedifferent crystal grains. The first active layer 11 and the secondactive layer 21 can be formed commonly to the R, G, and B pixels.However the first active layer 11 and the second active layer 21 canalso be formed differently for the R, G, and B pixels, so that a whitebalance can be maintained by making different crystal grains fordifferent colors on the second active layer 21 forming the driving TFT20 (this is not shown in drawings).

[0059] According to an embodiment of the invention, the first activelayer 11 and the second active layer 21 can be formed using apolycrystalline silicon thin film. The first active layer 11 and thesecond active layer 21 formed by the polycrystalline silicon thin filmcan be formed differently. In the embodiment of the invention shown inFIG. 1, the first active layer 11 and the second active layer 21 can beformed to have the crystal grains of different shapes. Here, it issufficient that the crystal grains on the channel areas of the firstactive layer 11 and the second active layer 21 have different shapesfrom each other, however, the crystal grains on the entire first andsecond active layers 11 and 21 have different shapes from each other dueto a complexity in designing the structure.

[0060] According to the embodiment of the invention, since the crystalgrains on the channel areas of the first active layer 11 of theswitching TFT 10 and the second active layer 21 of the driving TFT 20have different shapes, a current transferred from the driving TFT to thelight emitting device is reduced while having active layers have samesizes to achieve high resolution.

[0061] As described above, in the organic electroluminescence display,in order to form a TFT suitable for the high resolution, especially, forthe high resolution of a small size, an on-current of the switching TFTincreases and an on-current of the driving TFT decreases. In theinvention, the on-currents of TFTs are controlled by forming the crystalgrains on the active layers of TFTs to have different shapes. That is,the on-current of the switching TFT is increased and the on-current ofthe driving TFT is lowered by controlling the shapes of the crystalgrains on the active layers of the switching TFT and the driving TFT.

[0062] Therefore, the crystal grain shape on the active layer of theswitching TFT and the crystal grain shape on the active layer of thedriving TFT can be decided according to the current mobilities in thechannel area of the active layers. When the current mobility is large inthe channel area of the active layer, the on-current becomes large, andwhen the current mobility in the channel area is small, the on-currentbecomes small. Consequently, in order to achieve high resolution bylowering the on-current of the driving TFT, the current mobility in thechannel area of the driving TFT active layer should be controlled to belower than that in the channel area of the switching TFT active layer.

[0063] The difference between the current mobilities can be obtainedbased on the shape of crystal grains in the polycrystalline thin filmforming the active layer. In particular, the difference between thecurrent mobilities can be obtained according to the crystal grain shapeof the polycrystalline silicon thin film.

[0064] That is, the shapes of crystal grains on the first and secondactive layers 11 and 21 of the switching TFT 10 and the driving TFT 20can be decided by the current mobilities in the channel areas ofrespective active layers, since the on-current of a TFT can be increasedwhen the current mobility is large in the channel area of the activelayer, and the on-current of the TFT can be lowered when the currentmobility is small on the channel area.

[0065] Therefore, shapes of the respective active layers should becontrolled so that the current mobility in the channel area of thesecond active layer 21 in the driving TFT is lower than that of thefirst active layer 11 of the switching TFT for lowering the on-currenton the driving TFT. The difference in the current mobilities can beobtained based on the crystallization structure of the polycrystallinesilicon thin film forming the active layer. That is, the difference inthe current mobilities can be achieved by forming the respective layerson polycrystalline silicon thin films having different crystallizationstructures.

[0066]FIG. 2 is a view of various crystallization structures of apolycrystalline thin film forming the active layer of TFT. Thepolycrystalline silicon thin film can be formed by crystallizing anamorphous silicon thin film using a sequential lateral solidification(SLS) method. SLS method uses a fact that the crystal grain of thesilicon grows toward in a vertical direction at an interface between aliquid phase silicon region and a solid phase silicon region. A part ofthe amorphous silicon is melted by transmitting a laser beam using amask, and the grain grows toward the melted silicon part from theinterface between the melted silicon part and the non-melted siliconpart.

[0067] The crystallization structure in FIG. 2 can be obtained by usingdifferent masks for different regions when performing SLS method on thethin film.

[0068] In a first crystallization structure 61 with a stripe shape, aplurality of primary grain boundaries 61 a which are straight linesparallel to each other are formed, and a second grain boundary 61 b in avertical direction at the primary grain boundaries 61 a. Also, a lengthfor a direction of the crystal grain having the above grain boundarystructures is formed to be longer than the width of that crystal grain.The length may be at least about 1.5 times longer than the shorter sideor more.

[0069] The first crystallization structure 61 is formed by melting andcrystallizing the amorphous silicon thin film using a mask and a laserbeam transmitting area of stripe shape. When the active layer of TFT isformed on the first crystallization structure, a difference in currentmobility (see FIG. 3) can be achieved based on an angle of the primarygrain boundary 61 a with the direction of current flow in the channelarea of the active layer. That is, the current mobility is the largestwhen the primary grain boundary is perpendicular to the direction ofcurrent flow in on the channel area of the active layer, and the currentmobility is the smallest when the primary grain boundary 61 a isparallel with the direction of current flow in the channel area of theactive layer. Therefore, when the channel area of TFT active layer isformed on the first crystallization structure 61 to be perpendicular forthe primary grain boundary 61 a, high current mobility can be achieved.

[0070] The above relation can be described by resistance components formovements of a carrier. When an angle between the current flowingdirection with the primary grain boundary 61 a is 0° in the channel areaof the active layer, the direction of current flow is parallel with theprimary grain boundary 61 a, however, the direction of current flow isperpendicular to a plurality of secondary grain boundaries 61 b.Therefore, when the carrier moves, the moving direction of the carrieris perpendicular to the secondary grain boundary 61 b, thus increasingthe resistance components toward movement of the carrier and loweringthe current mobility.

[0071] On the contrary, when an angle of the direction of current flowin the primary grain boundary 61 a is 90°, the direction of current flowis perpendicular to the primary grain boundary 61 a, however, thedirection of current flow is parallel to a plurality of secondary grainboundaries 61 b. Therefore, the secondary grain boundary 61 b isparallel to the moving direction of the carrier when the carrier moves,thus reducing the resistance components toward carrier movement of thecarrier and increasing the current mobility.

[0072] The difference in the current mobilities causes the difference inon-currents. That is, as the angle made by the primary grain boundarywith the direction of current flow in the channel area of the activelayer is increased, the current mobility becomes larger, andaccordingly, the on-current also increases. Therefore, as describedabove, a channel area of the switching TFT requiring a high on-currentvalue can be designed to make an angle of about 90°, for example, withthe direction of current flow and not an angle of 0° for direction ofcurrent flow.

[0073] In a second crystallization structure 62, primary grain boundary62 a is formed as rectangle, and can be fabricated using a mask on whicha laser beam transmitting area of stripe shapes and a laser beamshielding area of dot shapes are mixed when performing the SLS method.When the active layer of TFT is formed on the rectangularcrystallization structure, a smaller current mobility value than that ofthe first crystallization structure 61 can be obtained.

[0074] A third crystallization structure 63 has very small sized andshapeless grains. The crystal grains in the third crystallizationstructure are formed using a flood radiation method in applying SLSmethod. A plurality of grain cores are formed by radiating laser overthe silicon without using a mask, and the grains grow to obtain thecrystal grains of fine and dense distribution, as shown in FIG. 2. Whenthe active layer of TFT is formed on the shapeless third crystallizationstructure, a smaller current mobility value than the current mobilityvalue in either of the above structures is obtained.

[0075]FIG. 4 is a view of a ratio of current mobilities when the activelayers are formed on the first, second and through third crystallizationstructures. As the current mobility can be changed according to theshape of crystallization structure, the switching TFT and the drivingTFT can be formed in various ways, as shown in FIGS. 5 through 7.

[0076] As shown in FIGS. 5 and 6, when the first active layer 11 of theswitching TFT is formed on the first crystallization structure 61, thesecond active layer 21 of the driving TFT may be formed on the secondcrystallization structure 62 or on the third crystallization structure63. Here, it is preferable that the primary grain boundary 61 a of thefirst crystallization structure 61 be disposed such that they areperpendicular to the direction of current flow on the channel area (C1)of the first active layer 11 which is formed on the firstcrystallization structure 61 to improve the current mobility. Accordingto the above structure, the current mobility in the channel area (C2) ofthe second active layer 21 is smaller than that of the first activelayer 11, and the on-current value of the driving TFT can be lowered.

[0077] Also, as shown in FIG. 7, when the first active layer 11 of theswitching TFT is formed on the second crystallization structure 62, thesecond active layer 21 of the driving TFT may be formed on the thirdcrystallization structure 63. As discussed above, the difference in thecurrent mobilities is generated due to the difference in thecrystallization structure, and the current mobility of the channel areaC2 of the second active layer is smaller than that of the first activelayer 11. Thus lowering the on-current value of the driving TFT islower.

[0078] It should be understood by one of ordinary skill in the art thatthe different crystallization structures of TFT active layers are notlimited to the above structures. That is, when the third crystallizationstructure for example, may be adopted for the active layer of TFTrequiring smaller current mobility between the switching TFT and thedriving TFT, the first or second crystallization structure is adoptedfor the active layer of TFT requiring larger current mobility. When thefirst crystallization structure is adopted for the active layer of TFTrequiring larger current mobility between the switching TFT and thedriving TFT, the second or third crystallization structure ma, forexample, be adopted for the active layer of TFT requiring smallercurrent mobility. It should be also understood by one of ordinary skillin the art that the invention is not limited to use of the showncrystallization structures. That is, different crystallizationstructures with different grain sizes may be used.

[0079] In addition, as shown in FIG. 8, the above effect can be obtainedby differentiating sizes of the crystal grains forming the channel areasof the respective TFT active layers. According to another embodiment ofthe invention shown in FIG. 5, the grain is crystallized using anexcimer laser annealing (ELA) method, and the sizes of grains aredifferentiated by radiating different energies to the switching TFT andthe driving TFT.

[0080] In ELA method, the sizes of crystal grains can be differentiatedaccording to densities of the radiated energy as shown in FIG. 9. Adifference between the sizes of crystal grains according to the energydensities of the laser in crystallizing the amorphous silicon thin filmof 500 Å in ELA method is shown in FIG. 9.

[0081] In FIG. 9, Region I represents a case that a partial melting isgenerated on the amorphous silicon by irradiating the amorphous siliconwith a laser with a relatively lower energy density, the crystal grainsgrow in a perpendicular direction due to the partial melting of theamorphous silicon to form the grains of small sizes.

[0082] Region II represents a case that a near complete melting isgenerated on the amorphous silicon by irradiating the amorphous siliconwith a laser with a relatively higher energy density, and the crystalgrains grow in a lateral direction from a few solid phase crystal germswhich are not melted to form the crystal grains of larger sizes.

[0083] Region III represents a case that a complete melting is generatedon the amorphous silicon by irradiating the amorphous silicon with alaser with the relatively highest energy density, and a plurality ofcrystal germs are generated by supercooling the melted silicon to growfine crystal grains.

[0084] Therefore, the size of crystal grain in Region II is the largest,then the size becomes smaller in order of Region I and then Region III.

[0085] In a case where the sizes of crystal grains are different fromeach other, the current mobilities according to the sizes are alsodifferent. That is, as shown in FIG. 10, the larger the size of crystalgrain is, the larger the current mobility is, thus forming a nearlystraight line graph.

[0086] As shown in FIGS. 9 and 10, when the crystal grain iscrystallized according to region II in which the largest grain can beformed, the largest current mobility can be obtained, and when thecrystal grain is crystallized according to region III in which thesmallest grain can be formed, the smallest current mobility can beobtained.

[0087] When the above result is applied to the embodiment of theinvention shown in FIG. 8, the first active layer 11 of the switchingTFT is formed on a fourth crystallization structure 64 having largercrystal grains, and the second active layer 21 of the driving TFT isformed on a fifth crystallization structure 65 having smaller crystalgrains. Then, smaller current mobility in the channel area of the secondactive layer 21 of the driving TFT can be obtained, and accordingly, theon-current value of the driving TFT can be lowered.

[0088] Therefore, generally if the fourth crystallization structure 64on which the first active layer 11 of the switching TFT is crystallizedin the region II of FIG. 10, the fifth crystallization structure 65 onwhich the second active layer 21 of the driving TFT is formed may becrystallized in Region I or in Region III of FIG. 10. Also, generally ifthe fourth crystallization structure 64 on which the first active layer11 of the switching TFT is formed may be crystallized in Region I ofFIG. 10, the fifth crystallization structure 65 on which the secondactive layer 21 of the driving TFT is formed may be crystallized inRegion III of FIG. 10.

[0089] The different crystallization structures are not limited thereto,and if the active layer of TFT requiring smaller current mobilitybetween the switching TFT and the driving TFT is crystallized in RegionIII of FIG. 10, the active layer of TFT requiring larger currentmobility may be crystallized in Region I or Region II. Also, if theactive layer of TFT requiring larger current mobility between theswitching TFT and the driving TFT is crystallized in Region II of FIG.10, the active layer of TFT requiring smaller current mobility iscrystallized in Region I or Region III.

[0090] As described above, when the crystal grains of different sizesare formed on the switching TFT 10 and the driving TFT 20 and the firstand second active layers 11 and 21 are formed thereon. The currentmobilities of the switching TFT and the driving TFT are differentiatedfrom each other, and the on-current value of the driving TFT 20 islowered to realize a high resolution.

[0091] On the other hand, respective sub-pixels of the organicelectroluminescence display device having the switching TFT and thedriving TFT have a structure shown in FIGS. 11 through 14.

[0092]FIG. 11 is a partially enlarged plane view of a sub-pixel amongthe pixels shown in FIG. 1, and FIG. 12 is a view of an equivalentcircuit for the sub-pixel shown in FIG. 11.

[0093] Referring to FIG. 12, the respective sub-pixel of the activematrix type organic electroluminescence display according to anembodiment of the invention comprises two TFTs such as a switching TFT10 for switching, a driving TFT for driving, a capacitor 30 and anelectoluminescence (EL) device 40. The number of TFTs and the number ofcapacitors are not limited thereto, and more TFTs and capacitors can bedisposed according to a design of desired device.

[0094] The switching TFT 10 is operated by a scan signal which isapplied to the gate line 51 to transfer a data signal which is appliedto the data line 52. The driving TFT 20 decides a current flowing intothe EL device 40 according to the data signal transferred through theswitching TFT 10, that is, voltage difference (Vgs) between a gate and asource. The capacitor 30 stores the data signal transferred through theswitching TFT 10 for one frame unit.

[0095] The organic electroluminescence display devices having thestructure shown in FIGS. 11, 13, and 14 are formed to realize the abovecircuit. As shown in FIGS. 11, 13, and 14, a buffer layer 2 is formed onan insulating substrate 1 made of glass, and the switching TFT 10, thedriving TFT 20, the capacitor 30, and the EL device 40 are disposed onthe buffer layer 2.

[0096] As shown in FIGS. 11 and 13, the switching TFT 10 includes a gateelectrode 13 connected to the gate line 51 for applying TFT on/offsignals, a source electrode 14 formed on the gate electrode 13 andconnected to the data line 52 for supplying the data signal to the firstactive layer, and a drain electrode 15 connecting the switching TFT 10with the capacitor 30 to supply power source to the capacitor 30. A gateinsulating layer 3 is disposed between the first active layer 11 and thegate electrode 13.

[0097] The capacitor 30 for charging is located between the switchingTFT 10 and the driving TFT 20 for storing a driving voltage required todrive the driving TFT 20 for one frame unit, and may include a firstelectrode 31 connected to the drain electrode 15 of the switching TFT10, a second electrode 32 formed to overlap the first electrode 31 on anupper part of the first electrode 31 and connected to a driving line 53through which the power source is applied, and an interlayer dielectriclayer 4 formed between the first electrode 31 and the second electrode32 to be used as a dielectric substance, as shown in FIGS. 11 and 13.The structure of the capacitor 30 is not limited to the above, forexample, the silicon thin film of TFT and the conductive layer of thegate electrode may be used as first and second electrodes, and a gateinsulating layer may be used as the dielectric layer.

[0098] As shown in FIGS. 11 and 14, the driving TFT 20 includes a gateelectrode 23 connected to the first electrode 31 of the capacitor 30 forsupplying TFT on/off signals, a source electrode 24 formed on an upperpart of the gate electrode 23 and connected to the driving line 53 forsupplying a reference common voltage to the second active layer 21, anda drain electrode 25 connecting the driving TFT 20 with the EL device 40for applying a driving voltage to the EL device 40. A gate insulatinglayer 3 is disposed between the second active layer 21 and the gateelectrode 23. Here, the channel area of the second active layer 21 ofthe driving TFT 20 has a different crystallization structure from thatof the channel area of the first active layer 11 of the switching TFT10, that is, the crystal grains of different shape or different size.

[0099] On the other hand, the EL device 40 displays a predeterminedimage information by emitting lights of red, green, and blue colorsaccording to the current flow. As shown in FIGS. 11 and 14, the ELdevice 40 includes an anode electrode 41 connected to the drainelectrode 25 of the driving TFT 20 for receiving positive power sourcefrom the drain electrode 25, a cathode electrode 43 disposed to coverthe entire pixel for supplying negative power source, and an organicemission layer 42 disposed between the anode electrode 41 and thecathode electrode 43 for emitting lights. Reference numeral 5 denotes aninsulating passivation layer made of SiO₂, and reference numeral 6denotes an insulating planarized layer made of acryl, or polyimide.

[0100] The above layered structure of the organic electroluminescencedisplay according to the embodiment of the invention is not limitedthereto, and the invention can be applied to any different structuresfrom the above.

[0101] The organic electroluminescence display having the abovestructure according to the embodiment of the invention can be fabricatedas follows.

[0102] As shown in FIGS. 13 and 14, a buffer layer 2 is formed on aninsulating substratel of glass material. The buffer layer 2 can beformed using, for example, SiO₂ and can be deposited using, for example,a plasma enhanced chemical vapor deposition (PECVD) method, anatmospheric pressure chemical vapor deposition (APCVD) method, a lowpressure chemical vapor deposition (LPCVD) method, or an electroncyclotron resonance (ECR) method. Also, the buffer layer 2 can bedeposited to have a thickness about 3000 Å.

[0103] An amorphous silicon thin film is deposited on an upper part ofthe buffer layer 2 to have a thickness about 500 Å. The amorphoussilicon thin film can be crystallized into the polycrystalline siliconthin film in various ways. Here, the crystallization to thepolycrystalline silicon thin film can be performed in such way that aportion where the switching TFT will be formed and a portion where thedriving TFT will be formed are classified, and the portion on which theswitching TFT will be formed is crystallized to have larger currentmobility and the portion on which the driving TFT will be formed iscrystallized to have smaller current mobility. Therefore, as describedabove, the area on which the switching TFT will be formed and the areaon which the driving TFT will be formed are crystallized to have thestructure shown in FIGS. 5 through 7 in a case where the crystallizationis performed using SLS method, and the area on which the switching TFTwill be formed and the area on which the driving TFT will be formed arecrystallized to have the structure shown in FIGS. 8 and 9 in a casewhere the crystallization is performed using ELA method. Also, the abovecrystallization structures can be formed in various ways.

[0104] After forming different crystallization structures, the firstactive layer 11 of the switching TFT 10 and the second active layer 21of the driving TFT 20 are patterned on the areas as shown in FIG. 1 toform the first active layer 11 and the second active layer 21 ofdifferent shapes.

[0105] After performing the patterning process of the active layers, thegate insulating layer is deposited on the patterned layers in PECVD,APCVD, LPCVD, or ECR method, and a conductive layer is formed using MoW,or Al/Cu and patterned to form the gate electrode. The active layer, thegate insulating layer, and the gate electrode may be patterned invarious orders and methods.

[0106] After patterning the active layer, the gate insulating layer, andthe gate electrode, N-type or P-type impurities are doped on the sourceand drain areas. As shown in FIGS. 13 and 14, after completing thedoping process, an interlayer dielectric layer 4 is formed, the sourceelectrodes 14 and 24 and the drain electrodes 15 and 25 are connected tothe active layers 11 and 21 through contact holes, and a passivationlayer 5 is formed. The layers may adopt various structures according todesign of the device.

[0107] On the other hand, the EL device 40 connected to the driving TFT20 can be formed in various ways, for example, an anode electrode 41connecting to the drain electrode 25 of the driving TFT 20 may be formedand patterned on the passivation layer 5 using, for example, an indiumtin oxide (ITO), and a planarized layer 6 may be formed on the anodeelectrode 41. In addition, after exposing the anode electrode 41 bypatterning the planarized layer 6, an organic layer 42 is formedthereon. Here, the organic layer 42 may use a low molecular organiclayer or a high molecular organic layer. In a case where the lowmolecular organic layer is used, a hole injection layer, a hole transferlayer, an organic emission layer, an electron transfer layer, and anelectron injection layer may be formed by being stacked in a single or acombination structure. Also, various organic materials such as copperphthalocyanine (CuPc), N,N-Di (naphthalene-1-yl)-N,N′-diphenyl-benzidine(NPB), and tris-8-hydroxyquilnoline aluminum (Alq3) can be used. The lowmolecular organic layer is formed using, for example, a vacuumevaporation method.

[0108] The high molecular organic layer may include the hole transferlayer and an emission layer. Here, the hole transfer layer is formedusing poly(3,4-ethylenedioxythiophene (PEDOT), and the emission layer isformed using a high molecular organic material such aspoly-phenylenevinylene (PPV)-based material or polyfluorene-basedmaterial in a screen printing method or in an inkjet printing method.

[0109] After forming the organic layer, the cathode electrode 43 may beentirely deposited using Al/Ca, or patterned. The cathode electrode 43may be formed as a transparent electrode in a case where the organicelectroluminescence display device is a front light emitting type. Anupper part of the cathode electrode 43 is sealed by a glass or a metalcap.

[0110] In above descriptions, the invention is applied to the organicelectroluminescence display device, however, the scope of the presentinvention is not limited thereto. The TFT according to the presentinvention can be applied to any display devices such as a liquid crystaldisplay (LCD), and inorganic electroluminescence display devices.

[0111] According to the invention, a current transferred from thedriving TFT to the light emitting device can be reduced without changingthe size of the active layer in TFT or the driving voltage, andaccordingly, a structure suitable for realizing the high resolution canbe obtained. A switching TFT having excellent switching properties canbe obtained, and at the same time, a driving TFT by which the highresolution can be realized can be obtained using properties of thepolycrystalline silicon. In addition, uniform brightness can be obtainedand life time degradation can be prevented using crystallizationstructures of TFT. Also, the aperture area is not reduced since there isno need to increase the length (L) of the driving TFT, and a reliabilityof TFT can be improved since there is no need to reduce the width (W) ofthe driving TFT.

[0112] While the invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention as defined by the following claims.

What is claimed is:
 1. A flat panel display comprising: a light emittingdevice; a switching thin film transistor including a semiconductoractive layer having a channel area for transferring a data signal to thelight emitting device; and a driving thin film transistor including asemiconductor active layer having a channel area for driving the lightemitting device so that a predetermined current flows through the lightemitting device according to the data signal, wherein the channel areaof the switching thin film transistor has crystal grains which aredifferent from crystal grains in the channel of the driving thin filmtransistor.
 2. The flat panel display of claim 1, wherein the channelarea of the switching thin film transistor and the channel are of thedriving thin film transistor have different current mobilities due tothe crystal grain associated with each.
 3. The flat panel display ofclaim 2, wherein the current mobility in the channel area of theswitching thin film transistor is greater than the current mobility inthe channel area of the driving thin film transistor due to the crystalgrains associated with each.
 4. The flat panel display of claim 1,wherein the crystal grains of the switching transistor have an averagesize which is different from an average size of the crystal grains ofthe driving transistor.
 5. The flat panel display of claim 4, whereinthe current mobility in the channel area of the switching thin filmtransistor is larger than the current mobility in the channel area ofthe driving thin film transistor due to the average size of crystalgrains associated with each.
 6. The flat panel display of claim 4,wherein between the switching thin film transistor and the driving thinfilm transistor, an average size of crystal grains in the channel areaof the thin film transistor requiring a larger current mobility islarger than an average size of crystal grains in the channel area of thethin film transistor with a lower current mobility.
 7. The flat paneldisplay of claim 4, wherein the average size of crystal grains in thechannel area of the switching thin film transistor is larger than theaverage size of crystal grain in the channel area of the driving thinfilm transistor.
 8. The flat panel display of claim 1, wherein thecrystal grains in the channel area of the switching thin film transistorhave a different shape than the crystal grains in the driving thin filmtransistor.
 9. The flat panel display of claim 8, wherein between theswitching thin film transistor and the driving thin film transistor thechannel area of the thin film transistor requiring a lower currentmobility have shapeless grain boundaries.
 10. The flat panel display ofclaim 9, wherein the crystal grains in the channel area of the thin filmtransistor requiring a larger current mobility than the current mobilityof the thin film transistor having the shapeless grain boundariesincludes substantially parallel primary grain boundaries, and secondarygrain boundaries extending in a substantially perpendicular directionfrom the primary grain boundaries between the primary grain boundaries,and the primary grain boundaries are formed as a stripe or a rectangle.11. The flat panel display of claim 8, wherein between the switchingthin film transistor and the driving thin film transistor, the crystalgrains in the channel area of the thin film transistor requiring highercurrent mobility include substantially parallel primary grainboundaries, and secondary grain boundaries which extend in asubstantially perpendicular direction from the primary grain boundariesbetween the primary grain boundaries and are arranged with an averageinterval which is shorter than an average interval of primary grainboundaries, the primary grain boundaries are formed to have stripeshapes, and the channel areas are arranged so that a flowing directionof the current is vertical for the primary grain boundaries.
 12. Theflat panel display of claim 11, wherein the channel area of the thinfilm transistor requiring lower current mobility than that of the thinfilm transistor having the primary grain boundaries of stripe shapesinclude at least one of shapeless grain boundaries and grain boundarieshaving primary grain boundaries of substantially rectangular shapes. 13.The flat panel display of claim 8, wherein between the switching thinfilm transistor and the driving thin film transistor, the crystal grainsin the channel area of the thin film transistor requiring higher currentmobility include substantially parallel primary grain boundaries, andsecondary grain boundaries extending substantially perpendicular fromthe primary grain boundaries between the primary grain boundaries, andthe primary grain boundaries have substantially rectangular shapes. 14.The flat panel display of claim 8, wherein the crystal grains in thechannel area of the driving thin film transistor have shapeless grainboundaries.
 15. The flat panel display of claim 14, wherein the crystalgrains in the channel area of the switching thin film transistor havesubstantially parallel primary grain boundaries and secondary grainboundaries extending substantially perpendicularly from toward verticaldirection for the primary grain boundaries between the primary grainboundaries, and the primary grain boundaries are formed as stripes orrectangles.
 16. The flat panel display of claim 8, wherein the crystalgrains in the channel area of the switching thin film transistor includesubstantially parallel primary grain boundaries and secondary grainboundaries which substantially perpendicularly from the primary grainboundaries between the primary grain boundaries and have an averageshorter interval than an average shorter interval of the primary grainboundaries, the primary grain boundaries are formed to be substantiallystripe shapes, and a direction of current flow in the channel area issubstantially perpendicular to the primary grain boundaries.
 17. Theflat panel display of claim 16, wherein the crystal grains in thechannel area of the driving thin film transistor have at least one ofshapeless grain boundaries and grain boundaries having primary grainboundaries of substantially square shapes.
 18. The flat panel display ofclaim 8, wherein the crystal grains in the channel area of the switchingthin film transistor have substantially parallel primary grainboundaries and secondary grain boundaries extending substantiallyperpendicularly from the primary grain boundaries between the primarygrain boundaries, and the primary grain boundaries are formed assubstantially square shapes.
 19. The flat panel display of claim 1,wherein the channel area of the active layer of the driving thin filmtransistor and the channel area of the switching thin film transistor isformed using a polycrystalline silicon.
 20. The flat panel display ofclaim 19, wherein the polycrystalline silicon is formed using acrystallization method using a laser.